Measurement of ultrasonic vibrations by means of fiber Bragg gratings (FBGs) that are embedded into the component under test or glued to the component surface are discussed, for example, in “Acousto-ultrasonic sensing using fiber Bragg gratings”, Daniel C Betz et al, 2003 Smart Mater. Struct. 12 122-128 or “Impact wave and damage detections using a strain-free fiber Bragg grating ultrasonic receiver”, Jung-Ryul Lee et al, NDT & E International Volume 40, Issue 1, January 2007, 85-93.